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<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  <input type="text" class="uname" value="张三">
  <input type="button" value="按钮">

  <script>
    var list = document.querySelectorAll('input')

    list[1].onclick = function() {  // 除了构造函数， 函数被谁调用， this指向谁
      list[0].value = '李四'

      this.disabled = true
    }
  </script>
</body>
</html>